The present invention relates to imagers that use one or more laser beams to expose material, e.g., for computer-to-plate (CTP) imaging to expose a printing plate.
Back-reflection is a known problem with laser-based computer-to-plate imagers exposing a film or photopolymer plate. Note that imagers for imaging plates are also commonly called imagesetters.
Many state of the art imagers are designed to process a wide variety of different plate types, not only from different vendors but also used for very different technical purposes. For example, Cyrel™ digital imagers made by Esko-Graphics NV of Gent Belgium, may be used for imaging film, imaging conventional polymer flexographic plates, and also imaging metal-backed polymer plates. Any one of these materials is referred to as a plate herein. Different types of plates typically might use different mechanisms to hold a plate onto the drum. Metal-backed plates for example, are preferably held onto the drum by permanent magnets embedded into the drum surface. Film plate and conventional computer-to-plate (CTP) polymer plates are preferably held onto the drum surface by vacuum, e.g., by vacuum applied from the inside of the drum to vacuum grooves and/or holes on the drum surface.
In many ablative plate and film imagers, problems arise from laser light not being absorbed by the layer of laser-light-sensitive ablatable material, called the “ablatable-layer” herein. This unabsorbed light can be reflected by the drum surface back to the rear side of the plate or film. This can cause several problems. A first problem is that back-reflected light can start undesired ablation or uncontrolled vaporization of the remaining ablatable-layer on the front side of the plate or film. A second problem is that the grooves and/or magnets on the surface of the drum, that is, variations in the surface property of the drum will affect the amount of back-reflected light either because of the variations in the drum surface absorption or because of variations relative amounts of reflected light and scattered light.
As an example of the second problem, suppose, for example, that image data is used that in a properly exposed plate would generate an image having a constant screen ruling. Suppose further that the grooves and/or magnets on the surface of the drum are regular structures. These structures cause changes of the back-reflected light, and as a result, instead of the image having a constant screen ruling, there may be, in addition, images that are similar to the regular variations on the drum surface caused by the grooves and/or magnets.
One common workaround is to use a laser whose laser radiation has high divergence. One example of such a laser is a multi mode laser diode. In such a case, the light from the laser will diverge so strongly that the back-reflected beam is not likely to have sufficient energy density to cause any ablation or other effect on the ablatable layer of the plate. This approach however has the disadvantage that the depth of focus for such a laser beam is very small. Consequently, the distance between any focusing optics used to focus the beam, and the plate surface has to be accurately maintained at a constant level, either by use of high mechanical accuracy or by an automatic focusing systems. In either case, the solution is relatively expensive.
Another solution is to use a use a drum whose surface is made from a material that absorbs radiation well. Unfortunately, most good absorbing materials such as black paint or anodized aluminium, might be, and likely will be ablated or discolored if exposed to a laser beam, so in time, the radiation absorbing property will be significantly reduced.